Many communication systems are based on a modulation of an electromagnetic field generated by a terminal. They range from a simple electronic tag used as a theft-prevention device to more complex systems where a transponder intended to communicate with the terminal, when present in the field thereof, is equipped with calculation or data processing functions (electronic purse, for example).
Electromagnetic transponder systems are based on the use of oscillating circuits comprising a winding forming an antenna, on the transponder side and on the terminal side. The antenna circuits of the transponder and of the terminal are intended to be coupled when the transponder enters the electromagnetic field of the terminal. These circuits are generally tuned to a same frequency corresponding to the excitation frequency of the oscillating circuit of the terminal.
So called remotely powered transponders, that is, transponders drawing from the high-frequency field radiated by the terminal an electric power capable of powering their electronic circuits, for example, a memory and/or a microcontroller, are here considered. A charge pump comprising a rectifying bridge and a storage capacitor is connected to the antenna circuit of the transponder. When the transponder is in the field radiated by the terminal, the electromagnetic signals provided by the antenna circuit supply the pump, and the capacitor charges. The capacitor plays the role of a power reservoir for providing a D.C. power supply voltage to the transponder circuits.
When the transponder must transmit data to the terminal, it modulates the load that it forms on the electromagnetic field radiated by the terminal. This process is generally called retromodulation. The retromodulation is carried out by means of an electronic switch, for example, a transistor, connected in parallel to the antenna circuit. When the switch is on, the charge pump is short-circuited and the transponder impedance drops. Thus, the most part of the power received by the transponder antenna is reflected towards the terminal. The impedance variation of the transponder translates as a current variation in the antenna circuit of the terminal. This variation is detected by an amplitude or phase demodulator of the terminal.
A disadvantage of this type of transponder is the fact that when the retromodulation switch is on, the charge pump stops being powered. Periods of data transmission by the transponder thus are critical in terms of generation of the transponder power supply.
It would be desirable to have a retromodulation system enabling the transponder to transmit data without interrupting the production of its electric power supply.